Nitrogen oxides, such as nitric oxide, nitrogen dioxide and the like are present in substantial quantities in the off gas produced during various industrial operations, such as nitric acid production. Previously these nitrogen oxides were simply discharged into the atmosphere, but modern air pollution standards now limit the quantities which may be discharged. Consequently, various processes have been developed to reduce or eliminate the nitrogen oxides in waste gas streams prior to their discharge into the atmosphere.
One such process uses catalytic combustion of the waste gases in a hot catalyst bed where reactions occur that destroy the nitrogen oxides. This process requires excess fuel to be added making it economically unattractive. Additionally the nitrogen oxides are destroyed, and hence cannot be recovered for subsequent use.
Another process has been reported whereby nitrogen oxides are removed from waste gas streams by contacting them with molecular sieves. These molecular sieves are normally in the form of granular solids which absorb the nitrogen oxides. The nitrogen oxides are subsequently desorbed under controlled conditions and recovered.
Bolme, in U.S. Pat. Nos. 4,053,555; 4,118,460; and 4,138,470 discloses a method for removing nitric oxide from gases by contacting the gases with aqueous solutions containing nitrate ions in sufficient concentration to oxidize the nitric oxide to nitrous acid and retain it in solution under stable conditions. The scrubbing solution is then regenerated under controlled conditions, producing a gas stream which contains the recovered nitrogen oxides in relatively high concentration and a scrubbing solution which can be recycled. The aqueous scrubbing solution can be nitric acid up to a concentration of 11.5 molar.